A. Field of the Invention
The present invention relates to a disk substrate for a perpendicular magnetic recording medium and a perpendicular magnetic recording medium mounted on an external storage device of a computer and other magnetic recording devices, in particular to a perpendicular magnetic recording medium suited for mounting on a hard disk drive (HDD), and a disk substrate for such a perpendicular magnetic recording medium.
B. Description of the Related Art
A perpendicular magnetic recording system is drawing attention as a technique to achieve high density magnetic recording, in place of a conventional longitudinal magnetic recording system. In particular, a double layer perpendicular magnetic recording medium is known to be suited as a perpendicular magnetic recording medium for achieving high density recording, as disclosed in Japanese Patent Publication No. S58-91. A double layer perpendicular magnetic recording medium is provided with a soft magnetic film called a soft magnetic backing layer under a magnetic recording layer that stores information. The soft magnetic backing layer, exhibiting high saturation magnetic flux density Bs, facilitates passage of the magnetic flux generated by a magnetic head. The double layer perpendicular magnetic recording medium increases intensity and gradient of the magnetic field generated by the magnetic head to improve recording resolution, and also increases the leakage flux from the medium.
The soft magnetic backing layer generally uses a Ni—Fe alloy film, an Fe—Si—Al alloy film, or an amorphous alloy film of mainly cobalt having a thickness in a range of about 200 nm to 500 nm formed by a sputtering method. To form such a relatively thick film by a sputtering method is not appropriate from the viewpoint of production costs and mass productivity. To solve the problem, use of a soft magnetic film formed by an electroless plating method has been proposed for the soft magnetic backing layer. Japanese Unexamined Patent Application Publication No. H7-66034, for example, proposes a soft magnetic backing layer using a NiFeP film produced by a plating method on an aluminum alloy disk substrate provided with a nonmagnetic NiP plating film. A CoNiFeP plating film formed on a glass substrate has been proposed (Digest of 9th Joint MMM/lntermag Conference, EP-12, p. 259 (2004)), as has a soft magnetic NiP plating film formed on an aluminum alloy disk substrate provided with a nonmagnetic NiP plating film (Digest of 9th Joint MMM/Intermag Conference, GD-13, p. 368 (2004)).
It is known that if the soft magnetic backing layer forms a magnetic domain structure and generates a magnetization transition region called a magnetic domain wall, the noise called spike noise generated from the magnetic domain wall degrades the performance of the perpendicular magnetic recording medium. Therefore, it is necessary to suppress of formation of the magnetic domain wall in a soft magnetic backing layer.
Since the NiFeP plating film is apt to form a magnetic domain wall, J. of The Magnetics Society of Japan, vol. 28, No. 3, p. 289-294 (2004) discloses that the magnetic domain wall formation must be suppressed by forming a MnIr alloy thin film on the plating film by a sputtering method. The suppression of the magnetic domain wall formation in the above-mentioned CoNiFeP plating film is described to be possible by conducting the plating process in a magnetic field. The soft magnetic NiP plating film is deemed not to generate spike noise.
Japanese Unexamined Patent Application Publication No. H2-18710 also proposes that the generation of spike noise can be suppressed by forming a backing layer composed of cobalt or a CoNi alloy having coercivity Hc in a range of 30 to 300 Oe in such a way as to exhibit magnetic anisotropy in the circumferential direction of the disk substrate. While the backing layer in this example is formed by a dry deposition process such as a sputtering method or an evaporation method, Japanese Unexamined Patent Application Publication No. H5-1384 proposes a method to form a Co—B film having Hc of at least 30 Oe and suppressing spike noise by a plating method, and suggests applicability to a soft magnetic backing layer.
For a disk substrate of a magnetic recording medium mounted on a hard disk drive, a glass disk substrate using crystallized glass or chemically reinforced glass is used as well as an aluminum alloy substrate provided with a nonmagnetic NiP plating film. The glass substrates, having high strength, are mainly used in a magnetic recording medium of a portable hard disk drive, which needs high chock resistance. The above-described plating method for forming a soft magnetic layer is effective to improve the productivity also in a glass disk substrate for a perpendicular magnetic recording medium.
The present inventors have discovered that the provision of the above-mentioned soft magnetic NiP plating film on a glass disk substrate is apt to orient the magnetization of the NiP plating film in the direction perpendicular to the film surface, generating very large noise, thus preventing the use of the soft magnetic NiP plating film for a soft magnetic backing layer. The same material of the NiP plating film can exhibit quite different magnetic properties and noise characteristics when used on an aluminum alloy disk substrate as opposed to the case where it is used on a glass disk substrate. Therefore, selection of a material used as a soft magnetic baking layer on a glass disk substrate is necessarily different from selection of a material used on an aluminum alloy disk substrate.
To suppress spike noise in the NiFeP plating film described previously, the magnetic domain wall formation needs to be suppressed by forming a MnIr alloy thin film on the plating film by a sputtering method. The requirement for provision of an additional film by a sputtering method to suppress the magnetic domain wall formation detracts from the merit of the plating method in production costs and mass productivity, and therefore is undesirable.
With the CoNiFeP plating film described above, it is difficult to apply a homogeneous magnetic field to the substrate in the plating bath in an actual manufacturing process, and is also very likely to impair the mass productivity. Although an iron-containing plating film, exhibiting high Bs, is suitable for a soft magnetic backing layer, ensuring the stability of a plating bath is known to be generally difficult because iron ions takes stable forms of both divalent and trivalent ions. So the iron-containing plating film is also defective in mass productivity.
The inventors have studied the correlation between coercivity and magnetic domain wall formation of the soft magnetic underlayer formed by a plating method, and found that a coercivity value of the plating film of not smaller than 30 Oe cannot completely prevent the magnetic domain wall formation, although some tendency of suppression was observed. It has been further determined that the increase of the coercivity deteriorates the read-write performance.
μs described above, the conventional technology does not achieve a backing layer on a glass disk substrate of a perpendicular magnetic recording medium that allows high density recording and suppresses spike noise while still being accompanied by low production costs and satisfactory mass productivity.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.